Optical integrated circuits such as optical sensor chips require packages that are more restrictive than normal integrated circuit chips. Consider the packaging of a chip having an imaging array of the type used to construct cameras. The package must have a clear window that will not interfere with the imaging of a scene onto the sensor chip that is internal to the package. Since the packages must be made of a plastic material for reasons of cost, this window must be protected from scratching in subsequent handling and fabrication steps in which the packaged chip is incorporated into a more complex optical system.
Many optical systems in which the package chips are utilized require that the chip be located at a precise position relative to a lens assembly that focuses a scene onto the surface of the chip. In practice the packaged sensor is mounted on some carrier such as a printed circuit board. The fabrication process must rely on the outside surfaces of the package, or the leads extending from the package, to position the chip on the carrier at the proper location relative to other components, such as the lens assembly. Hence, the chip must be mounted at a precise location in the packaging material relative to the outside surfaces of the package. In addition, the outside surfaces must have a greater degree of uniformity than the surfaces normally provided in conventional integrated circuit packages.
In addition, the packages must be light-tight in those areas other than the clear window through which the image is received. Conventional integrated circuit packages are constructed using a single plastic encapsulant. Since a clear area is needed, this encapsulant must be clear and capable of being molded to provide a defined surface through which the image is received. As a result, the sides and back of the package must be further processed to provide the required light-tight enclosure. These additional operations increase the cost of the final packaged sensor.
Finally, the plastics and molding equipment used in the packaging of integrated circuits are not easily adapted to providing optical quality windows in the normal packages. Conventional IC packaging molds do not provide optically flat surfaces or the lens features of the type needed for imaging sensors. The dimensional characteristics of the package depend on the precision of the mold. Molds used in plastic molding operations require periodic polishing to restore the mold surfaces after a large number of parts have been produced. The size of the mold cavity is altered by the polishing operation, and hence, the size of the parts produced by the mold changes slightly with each polishing operation to which the mold is subjected. While these changes are not significant for normal IC packages, the dimensional changes can be greater than the required tolerances for optical packages.